It is helpful in the operation of internal combustion engines to know the pressure in a combustion chamber, and more specifically in an engine cylinder. For example, this pressure can be used to control ignition timing, thus allowing the engine to obtain better fuel consumption results. Combustion chamber pressure can also be used to detect engine knock.
It is known in the art that a pressure sensing device, such as a piezoelectric force ring, may be used to indicate combustion chamber pressure. A description illustrating such an arrangement is found in U.S. Pat. No. 4,153,0l9 to Laubenstein et al, issued May 8, 1979. In the disclosure of that patent a cylinder head bolt secures a force ring to the engine. This enables the force ring to detect loads that are transferred to the cylinder head bolt through the cylinder head as a result of combustion chamber pressure.
Although force rings secured under head bolts can detect loads acting on the cylinder head as a result of combustion chamber pressure, they also detect extraneous forces, such as thermal expansion and external shock or inertia loads. This event often occurs when the extraneous forces are of comparable magnitude to the load being measured from combustion chamber pressure. Consequently, these extraneous forces must be filtered away so that the sensor provides a clean output signal. Thermal stress, which has low frequency characteristics, is easily filtered away without affecting the signal. Filtering high frequency shock noise, however, introduces a phase lag that may cause inaccuracies in the timing of engine control operations. In addition, such an arrangement provides a signal that is non-linear due to the lack of elasticity of the head gasket, which is variably loaded as combustion chamber pressure moves the engine head relative to the engine block. It is therefore desirable to provide a linear pressure sensing arrangement which, compared to a headbolt sensor, is comparably more sensitive to combustion chamber pressure than to extraneous forces, is more linear, and introduces no phase or time lag.
An advancement toward that goal is shown in the sensor of U.S. Pat. No. 4,601,196 to Frelund, issued July 22, 1986. The sensor of the Frelund patent utilizes a pressure sensing device that has a main body and a probe. The probe engages a wall that flexes in direct response to varying combustion chamber pressure. The body engages a different wall that is relatively fixed. The result is that when combustion chamber pressure changes, the probe will move relative to the body and thus generate a signal. This type of arrangement may be implemented through a coolant passage of the engine cylinder head, with the body retained in an upper wall and the probe extending through the coolant passage to a lower wall forming part of the combustion chamber. If the sensor is properly placed, the relative motion between the upper and lower walls due to varying combustion chamber pressure places a load on the sensor that is much greater than any loads caused by extraneous forces. This greatly increases the signal to noise ratio of the sensor output.
It is preferable, however, that the sensor is placed near the center of the cylinder relative to the other cylinders. This location produces a stronger signal than other locations because the cylinder head wall flexes more near the center than at the sides. Additionally, this location also produces a linear signal that is free from interference from adjacent cylinders. In some situations, however, the sensor described in the Frelund patent cannot be placed near the center of the cylinder. A common example illustrating this point is an engine having four valves for each cylinder with a spark plug opening centrally located between the valves. This would require using an alternate location which would provide a weaker signal and might subject the sensor to interference from adjacent cylinders. Thus, it is advantageous to have a sensor that can be placed near the center of the cylinder in these situations.
One arrangement in the prior art that can be used to locate a sensor near the center of a cylinder is described in U.S. Pat No. 4,602,506 to Sawamoto et al, issued July 29, 1986. The Sawamoto et al patent shows an arrangement where an annular pressure sensor is clamped to a spark plug seat by the spark plug itself. However, although the spark plug may be centrally located at the top of the combustion chamber, the Sawamoto et al sensor measures the strain that is placed on the spark plug threads rather than the movement between a wall subject to combustion chamber pressure and a fixed wall. This arrangement, then, does not provide the advantages of the aforementioned Frelund et al sensor. In particular, its output is lower and tends to be non-linear. In addition, the annular sensing element is loaded by the spark plug itself. Therefore it must work with the same axial preload and is further subject to torsional loads by the rotation of the spark plug. It is also a burden to have the sensing arrangement disturbed when a spark plug has to be replaced.
Therefore, in conclusion, it is desirable to have a combustion chamber pressure sensing arrangement located where combustion chamber pressure loads are of much greater magnItudes than the surrounding extraneous forces. Ideally, the sensing arrangement should be placed near the center of the cylinder to insure that the sensor produces a strong linear signal and is minimally influenced by adjacent cylinders. Furthermore, the sensing element should be easy to install, separated from other engine components for independent operation and servicing, and free from torsional loads that may cause the failure of the sensor.